Would the investor have to use an assignment contract between him and new end buyer to collect the mark-up fee aka assignment fee at closing?

It offers a lot more than just rent collection.

Collect properties instead of money.

Also, focus on 2 years of job/income stability.Class D Properties:Cashflow vs Appreciation: Typically, all cashflow with little, maybe even negative, relative rent & value appreciationVacancy Est: 20%+ should be used to cover nonpayment, evictions & damages.Tenant Pool: majority will have FICO scores under 560 (almost 30% probability of default), little to no good tradelines, lots of collections & chargeoffs, recent evictions.

Perhaps if your partner was a general contractor or handyman type there may be some merit there to reduce costs / increase profitability.If your partner enjoys deal sourcing, a better move might be for him to become an agent - so he could legally collect commissions from finding deals and managing properties for multiple people. 

But I wanted to collect some recommendations in the meantime. 

Here's the deal: You don't need to withdraw and return the current security deposit just to collect the additional $100.

The withholding can also be stopped altogether, and only actual taxes owed will be collected by filing form MW506NRS at least 21 days prior to settlement and filing it with the Comptroller of Marylands office to get the exact actual tax owed calculated. 

Many opt for other investing methods in their SDIRA like private lending, crypto, private equity...anything except collectibles and life insurance.Note that a Solo 401k is exempt from UBIT on leveraged real estate so if you have self employment activity and no full time employees save for you and your spouse you should consider the Solo K as an option. 

Also, focus on 2 years of job/income stability.Class D Properties:Cashflow vs Appreciation: Typically, all cashflow with little, maybe even negative, relative rent & value appreciationVacancy Est: 20%+ should be used to cover nonpayment, evictions & damages.Tenant Pool: majority will have FICO scores under 560 (almost 30% probability of default), little to no good tradelines, lots of collections & chargeoffs, recent evictions.